The invention relates to a wavelength division multiplexed (MDW, hereinafter) optical communication system, and especially to a level-flattening circuit for WDM optical signals for maintaining optical powers of repeating optical signals at a constant level throughout whole wavelengths.
In a conventional technology disclosed in Japanese Patent Kokai 7-30520, n (n is an integer) optical transmitters generate n optical signals having n different wavelengths xcex1 to xcexn and these optical signals are respectively attenuated by n optical attenuators corresponding wavelengths of xcex1 to xcexn, multiplexed by a wavelength division multiplexer, amplified by an Er-doped optical fiber, and transmitted therefrom. In most cases, the optical signals are plus code modulated.
A part of optical energy of the WDM optical signals is split by an optical splitter, and demultiplexed by a wavelength division demultiplexer into n optical signals having the wavelengths of xcex1 to xcexn, which are respectively detected by optical to electrical converters (O/E converters, hereinafter) corresponding to the wavelengths of xcex1 to xcexn. The aforementioned optical attenuators corresponding to the n optical signals are respectively feedback controlled based on the outputs of the n O/E convertors.
In this way, powers of the WDM optical signals amplified by the Er-doped optical fiber amplifier are maintained at a constant level throughout the whole wavelengths.
However, the aforementioned level-flattening circuit for the WDM optical signals is designed as an apparatus to be used in an optical signal transmitting station, which is provided with plural optical transmitters for generating plural optical signals having the wavelengths of xcex1 to xcexn, and cannot be used in an optical signal repeating station.
If the principle of the aforementioned level-flattening circuit for the WDM optical signals is directly applied to the same used in the optical signal repeating station, the individual optical signal levels in the optical signal repeating station must be transmitted to the optical signal transmitting station in order to constitute a feedback loop, the system is magnified, and a statistactry result cannot be obtained.
Accordingly, it is an object of the invention to solve problems in the aforementioned level-flattening circuit for WDM optical signals, and provide a level-flattening circuit for WDM optical signals which can be applied to a optical signal repeating station.
According to a feature of the invention, a level-flattening circuit for WDM optical signals comprises;
A wavelength division demultiplexer, which is supplied with the WDM optical signals and demultiplexing them into individual optical signals having different wavelengths,
optical attenuators provided for the individual optical signals supplied from the wavelength division demultiplexer,
optical splitters, which split the individual optical signals supplied from the optical attenuators at a predetermined rate,
photodiodes for converting the split optical signals supplied from the optical splitters into electrical signals,
a control circuit, which feedback controls attenuations of the optical attenuators corresponding to the photodiodes so that the electrical signals outputted from the photodiodes are maintained at respective predetermined levels, and
a wavelength division multiplexer for multiplexing the individual optical signals passed through the optical splitters.